This is bad in the case of online orders, where the order will usually go through anyway (vendors want to sell even when you don’t confirm your email address, and don’t care a lot about someone else getting the notification emails), because if by mistake you registered your email address as new although you already had an account with that address, the order won’t get associated with your account. Or if it does automatically get associated with the account (or after confirmation), then that’s bad as well if you really entered someone else’s email address, because your order will then get associated with their account.
This is the reason why attempting to sign up for an existing account generally fails right away, at least for online retailers.
Not sure exactly what you mean? Are you referring to a purchase flow where you are buying something and also given the option to checkout as guest or signin/signup?
I am only speaking to the typical signup flow that anyone can access signed out without putting in any information besides email/pw. If you are in a purchase flow where valid credit card info is already entered and is going to result in actual purchase $$, you've already excluded bots/hackers who would be trying to brute force account enumeration. It would be totally fine to confirm the existence of an account in a web form in this situation as it is not a flow that can be easily brute forced for free with little effort or info needed (like a credit card).
If security is not a priority for the vendor, then yes. Otherwise, no, the order will not go through anyway.
I'm very aware that in many many cases, the former is true in the real world, but that doesn't change the fact. This isn't a good justification for the op's dismissal of the practice.
Woocommerce plug-in of Wordpress solved that issue. You can checkout as a guest with someone else’s email, you won’t see other orders from that email and if you do want to see them then you need to sign up for an account
This is the reason why attempting to sign up for an existing account generally fails right away, at least for online retailers.